It seems to me that every other Disney movie has either been made into a live action movie(Sleeping Beauty, Cinderella, Snow White, etc.) or fancast for one. However I haven't seen one for Brave.
So here is mine...
Young Merida - Francesca Capaldi
Most people would argue that at age 10, Francesca Capaldi, is to old to play young Merida, I think she is a good fit. She has the incredibly curly red hair to pull off the look.
Queen Elinor - Ashley Judd
Ashley Judd is the perfect fit for Queen Elinor, in my opinion. To me they have very strong facial similarities. I remember watching Divergent and telling my sister that Tris's mom looked like the mom from Brave.
King Fergus - Vinnie Jones
Vinnie Jones has the brawny tough guy look to him, I say slap a ginger wig on him and call him the Bear King
The Triplets - Ocean Maturo
Yes Ocean is just one kid, where there should be three but there are several movies and tv shows where one child plays twins so why can't he play triplets. Also Ocean's curly locks are blonde instead of fiery orange, but with some washable hair dye, I think this 3 year old would be the perfect fit.
Merida - Bella Thorne
Bella Thorne isn't a perfect fit, her hair is to pale and lacks the abundance of curls, but with a wig or hair dye and several hours with a curling iron I think she could make it work.
Lord Macintosh - Gwaine Merlin
I think this one just speaks for itself
Young Macintosh - Mateo Arias
If you've ever seen Kickin' It on Disney XD you will see a remarkable similarity in Young Macintosh and Mateo's character Jerry's personalities. Now that he let his hair grow long I think he is a perfect fit.
Lord Dingwall - Billy Crystal
I can just see him play a grumpy old man
Wee Dingwall - Cole Sprouse
He's got the goofy smile, the crazy hair, cold there be anyone better?
Lord Macguffin - Richard Riehle
He is a little old but I think he has the right look to him.
Young Macguffin - Andrew Caldwell
He's the right size and has the right look to his face. But can he babble non since?
Maudie, the castle maid - Melissa McCarthy
Melissa McCarthy seems to be perfect in any role, I can definitely see her in the part of the overzealous maid who frightens easily.
The Witch - Carol Kane
For this one I included a picture of her as the witch in the Princess Bride, so I think she pretty much has this role in the bag